1. Field of the Invention
The present invention relates to a nitride semiconductor light-emitting device which uses as a light emitting layer an indium-containing group-III nitride semiconductor layer of a multi-phase structure composed of a main phase and sub-phases which have different indium contents, and a method of fabricating the same.
2. Description of the Prior Art
An indium-containing group-III nitride semiconductor expressed by the general formula AlxGayInzNaM1xe2x88x92a(x+y+z=1, 0xe2x89xa6x, y less than 1, 0 less than zxe2x89xa61, 0 less than axe2x89xa61, M: group-V element other than nitrogen) is used as a light emitting layer for nitride semiconductor light-emitting devices which radiate a short wavelength light. Particularly, a gallium indium nitride mixed crystal (GabIn1xe2x88x92bN: 0xe2x89xa6b less than 1) is a typical component material of the light emitting layer (see Japanese Patent Publication No. 55-3834). To mention one background art, a gallium indium nitride mixed crystal (Ga0,80In0.20N) having an indium content of 20% has been used as a light emitting layer of a blue light emitting diode (LED) emitting light of a wavelength of about 450 nm. A gallium indium nitride mixed crystal having an indium content of 45% has been used as a light emitting layer of a green light emitting diode (LED) emitting a light of a wavelength of about 525 nm.
The gallium indium nitride mixed crystal is also used as a well layer of a single or multi-quantum well structure which serves as a light emitting part thereof (see Japanese Patent Public Disclosure No. 9-36430). It has been considered desirable for the gallium indium nitride mixed crystal heretofore widely used as a light emitting layer to be homogeneous regarding In content (see Japanese Patent Public Disclosure No. 9-36430). However, it was recently found that a gallium indium nitride layer which is inhomogeneous regarding the In content is advantageous as a light emitting layer (see Japanese Patent Public Disclosure No. 10-107315). This so-called multi-phase structure gallium indium nitride is a layer formed of an aggregation of phases having different In content.
The high-intensity light emission from the gallium indium nitride layer of the multi-phase structure having unevenness of the In content and exhibiting xe2x80x9cvariancexe2x80x9d of the In content as described above has been concluded to be attributable to quantized light emitters such as quantum dots. A gallium indium nitride layer of the multi-phase structure is generally composed of a main phase (matrix phase) and sub-phases, the main phase accounting for the major portion thereof in terms of volume (see Japanese Patent Public Disclosure No. 10-56202). The sub-phases are usually different from the main phase in In content. Even among the sub-phases, the In content always differs. The sub-phases take the form of approximately spherical or island-shaped microcrystals, which are scattered in the main phase.
In the background art, a participation of a quantized level, which is formed at a boundary between the microcrystal (sub-phases) and the main phase surrounding it, in a light emission is indicated (see Japanese Patent Public Disclosure No. 10-107315). The size of the microcrystal is several nm to several tens of nm in diameter, and the microcrystal is large enough to act as a quantum dot. It is considered that these sub-phases act as a quantum dots taking part in light emission from a light emitting layer composed of an indium-containing group-III nitride semiconductor.
Although the sub-phases (quantum dots) participate in the light emission from the light emitting layer formed of the indium-containing group-III nitride semiconductor, no technology is available for obtaining stable emission characteristics, i.e., stable emission intensity and emission wavelength, of the light emission from the light emitting layer. The emission characteristics obtained are therefore unstable.
The principal reason why stable emission characteristics have not been obtained in the background art is that the requirements for operating the sub-phases as quantized light emitter are not well known. The requirements for effectively operating the microcrystals, i.e., the sub-phases, as quantum dots must be clarified in order to obtain stable emission characteristics, especially at high emission intensity.
The present invention was made in view of the above described circumstances, and has as an object to provide a nitride semiconductor light-emitting device which is capable of providing stable and excellent emission characteristics of light from a light emitting layer, by elucidating the requirements for effectively operating the sub-phases as quantum dots, and a manufacturing method of the same.
To achieve the foregoing object, the present invention defines the structure of a region near the boundary between each sub-phase and the main phase surrounding the sub-phases, in order to allow the microcrystals constituting the sub-phases to act as quantized light emitters. Specifically, a nitride semiconductor light-emitting device of the present invention having an indium-containing group-III nitride semiconductor layer as its light emitting layer, which has a multi-phase structure composed of a main phase and sub-phases having different indium contents, is characterized in that the sub-phases are principally formed of crystals whose boundaries with the main phase are surrounded by strained layers.
In the method of fabricating a nitride semiconductor light-emitting device of the present invention having an indium-containing group-III nitride semiconductor layer as its light emitting layer, which has a multi-phase structure composed of a main phase and sub-phases having different indium contents, after the light-emitting layer is subjected to a heat treatment in a range between 950xc2x0 C. and 1200xc2x0 C., the light-emitting layer is cooled from the heat treatment temperature to 950xc2x0 C. at a rate of 20xc2x0 C. per minute or more, and is cooled from 950xc2x0 C. to 650xc2x0 C. at a rate of 20xc2x0 C. per minute or less, thus forming strained layers at the boundaries between the sub-phases and the main phase constituting the light-emitting layer.
As described above, in the present invention, since each sub-phase is mainly composed of a crystal having a strained layer at a boundary with the main phase around it, the strained layer present around the sub-phase stably generates carriers which increase the emission intensity. Therefore, the crystal constituting the sub-phases is enabled to effectively act as a quantized emission medium, so that a short wavelength visible light emitted from the nitride semiconductor light-emitting device including this light emitting layer can be made to exhibit a high emission intensity and excellent monochromaticity.
These and other objects and features of the present invention will be more fully apparent from the following detailed description taken in conjunction with the accompanying drawings.